Microstructure-induced phonon focusing effects and opportunities for improved material quantification

It is well known that single-crystal materials such as silicon have anisotropic elastic properties which depend on crystalline direction, causing the characteristic properties of a propagating elastic wave to have spatial and directional dependencies. As a result, variations in the speed and energy flux of an elastic waves propagating in a single crystal material typically produce spatial patterns, which can be used to infer the internal structure of a crystalline material. For polycrystalline materials, similar effects can be manifested when textured or single phase, equiaxed grains are involved, and coherent wave interference processes exist. Three examples of this are presented in this paper, where the propagation of longitudinal waves within single crystal silicon, textured titanium, and polycrystalline nickel materials are characterized using scanning laser vibrometry in a thru-transmission detection mode. By measuring and studying the resulting patterns, it is anticipated that inversion methods can be developed for the quantitative evaluation of single crystal and polycrystalline materials.